1. Field of the Invention
The present invention generally relates to an optical disk reproducing device and its seek operation method, and more particularly, to an optical disk device for reproducing an optical disk of a constant-linear-velocity (CLV) recording system of CD-ROM, etc., and its seek operation method for transiting tracks of the disk by transition of the optical pickup.
2. Description of the Prior Art
In the optical disk which is called CD-ROM, (which will be designated "disk" hereinbelow), a digital signal is generally recorded by a modulation system which is called eight-to-fourteen modulation (EFM), in the same way as a compact disk (CD) for audio devices.
In the CD-ROM, a time length for one bit and one frame, and a recording length on the disk, in an inside tracking part thereof, is the same as that in an outside tracking part of the disk. Therefore, in a conventional CD-ROM reproducing device, a rotation rate of the disk is changed in relation to a position in a radial direction of the disk so that a pickup could scan the disk whereby each scanned position moves at a constant linear velocity.
As mentioned above, in the CD-ROM reproducing device with the CLV system, the rotation rate of the disk needs to be controlled to a proper rotation rate corresponding to the position in the radial direction of the disk. To meet the need, when the optical pickup seeks a target address, the rotation rate of a spindle motor must be changed from the rotation rate for the present address to that for the target address.
FIG. 1 shows time charts for explaining the seek operation in the conventional CD-ROM reproducing device with the CLV system.
In the conventional CD-ROM reproducing device, for accessing any address on the disk to reproduce the data at the address, after the optical pickup is reached near the target track, the rotation rate of the disk is controlled by use of the EFM signal which is read out from the data near the target track. Therefore, for the seek operation for which the optical pickup is transited to access the other track, it takes the transition time of the pickup and the control time of the disk to reach the proper disk rotation rate, and there is thus the problem of a long time required for the seek operation.
An example of the above-mentioned operation is shown as EX. 1 in FIG. 1. At time t.sub.0, the pickup starts to transit. At time t.sub.1, the pickup stops transiting. After that, the rotation rate of the disk is adjusted to the specified linear velocity by the EFM signal which is read out. From time t.sub.3, the data can be reproduced by synchronizing to the EFM signal being read out. Therefore, it takes a long time (t.sub.1 to t.sub.3) to achieve the proper disk rotation rate, which thus causes the seek time to increase.
Alternatively, systems in which the disk rotation rate is controlled during transition of the pickup, have been proposed. For example, disk rotation rates for positions in the radial direction of the disk can be previously memorized. Thereby, during the seek operation, the disk rotation rate can be controlled to the proper rotation rate for the target track during the transition of the optical pickup. For another example, the disk rotation rate can be speeded up/slow down by the time corresponding to the distance of the pickup transition.
However, for the purpose that the data can be reproduced with synchronization to the EFM signal which has been read out by the optical pickup after the transition of the optical pickup, the disk rotation rate can be adjusted to within a few percent of the proper rotation rate for the target track during the transition, but there is still a problem that the seek time is increased by the adjusting time to exactly achieve the proper rotation rate.
An example of the above-mentioned operation is shown as an EX. 2 in FIG. 1. In this example, the rotation rate is controlled during the pickup transition during the time t.sub.0 to t.sub.1. However, after the time t.sub.1, the disk rotation rate is further controlled to the specified linear velocity by using the EFM signal which has been read out, and at the time t.sub.2 the data can be reproduced with synchronization to the EFM signal which has been read out. Therefore, in this operation it takes time t.sub.1 to t.sub.2 for adjusting the disk rotation rate within a few percent of the proper rotation rate, so that the seek time is increased by that time.
Furthermore, a system in which the disk is controlled with a constant angular velocity (CAV) instead of the CLV is also proposed. However, in this system, the difference of the bit rate between the inside track and the outside track of the disk is 2.5 times at a maximum value, and if the upper limit of the data transmission speed is equivalent to that of the CLV system, there is a problem that the average of the data transmission speed is decreased.